Sealing cover unit for a container for a semiconductor device

ABSTRACT

A cover unit for hermetically sealing a container for a semiconductor device comprises a rectangular metallic cover element such as a cobalt-nickel-iron alloy and a plurality of contiguous turns of wire of a heat-fusible material such as a gold-tin eutectic alloy formed and disposed on the cover element with the outermost turn substantially in registry with the periphery thereof, said turns being spot-welded thereto substantially at the corners of the cover element. The preferred method of fabricating the foregoing hermetic sealing cover unit comprises disposing it in a shallow cavity of a rotatable nonconductive supporting member, such cavity having dimensions to retain the cover element in desired position and forming a flat single-layer multiturn wire winding of heat-fusible material by winding it on a mandrel in contact with the cover element and rotatable therewith and spot-welding the winding to the cover element at a plurality of spaced points. An apparatus for performing the foregoing method comprises a rotatable nonconductive supporting member having a shallow cavity for receiving and closely fitting a cover element, a mandrel rotatable with and engaging the supporting element and having an annular recess proportioned to receive a single-layer multiturn wire winding and having an extending supporting shaft, means for rotating the supporting member and the mandrel together with means for feeding wire to the mandrel to form such single-layer multiturn winding. The apparatus also includes a nonrotatable electrode assembly disposed about the shaft and normally disengaging the cover element but movable axially of its shaft to spot-weld the multiturn winding to the cover element.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The sealing cover unit of the present invention is of particular utilityin hermetically sealing a container for a semiconductor device such asan integrated circuit which must be protected from exposure to ambientatmospher.

2. Description of the Prior Art

In the manufacture of semiconductor devices, there has been arequirement for hermetically sealing the container is which the activesemiconductor device is housed, usually a cavity in a metallic orceramic body. For this purpose, a metallic cover and a solder preformring have been used. Initially, the metallic cover and the solderpreform ring were assembled separately and disposed on a sealing ring ofa container and the assembly was heated to solder the cover to thecontainer and hermetically seal the container.

In accordance with current general practice, the solder preform ring ispreattached to the sealing cover and the resulting unit disposed overthe container of the semiconductor device and heated to fuse the solderpreform to the cover and to the container as described and claimed inApplicant's U.S. Pat. Nos. 3,823,468 and 3,874,549. Such preattachedsealing cover-solder preform units have acquired substantial commercialsuccess since they have several important advantages over the use ofseparate sealing covers and solder preforms, namely, increasingsubstantially the hermetic sealing yields, lowering assembly costs, andalleviating the logistics of keeping balanced inventories of the sealingcovers and the solder preforms for various sizes of semiconductorpackages.

In the current practice of fabricating the preattached cover element andsolder preform, the latter usually has a thickness of 0.001 inch to0.005 inch and is stamped from a continuous solder ribbon of the desiredgold-tin eutectic alloy. As a result, approximately 15% of the ribbonresides in the solder preform and 85% becomes scrap in the form of askeleton and the center punched-out sections. Ordinarily, the gold orother precious or semiprecious metals in the scrap are recycled bymelting, adjusting the alloy content to the required ratio, recasting,rolling and reusing for manufacturing additional solder preforms. Often,the alloy becomes unusable for recycling because of the accumulation offoreign materials. The solder must then be refined into its various puremetal components. A so-called virgin melt must then be made from therefined metals.

Such recycling and refining operations entail appreciable labor,material, and energy costs. Some of the precious metal is necessarilylost in the refining and remelting steps of manufacture and this addsadditional cost. A substantial economic disadvantage of obtaining such alow yield of the solder ribbon in the finished solder preform is therequirement that an excessive amount of precious metal--usuallygold--must be tied up in inventory and work in process. The interestcosts on the excessive amount of precious metal that must be purchasedare substantial.

It is an object of the present invention to provide a composite sealingcover-solder preform unit which is substantially free from solder scrapand which reduces the cost of preassembling and preattaching the coverand the solder preform.

SUMMARY OF THE INVENTION

In accordance with the invention, a cover unit for hermetically sealinga container for a semiconductor device comprises a metallic coverelement and a plurality of turns of wire of a heat-fusible materialformed and disposed on the cover element with the outermost turnsubstantially in registry with the periphery thereof and the turns beingfused thereto at a plurality of spaced points.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription, taken in connection with the accompanying drawings, whileits scope will be pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view, partly in section, of a sealing cover unitfor a container for a semiconductor device embodying the invention;

FIG. 2 is a schematic view of an apparatus for fabricating the hermeticsealing cover unit of FIG. 1;

FIG. 2A is a perspective view of the electrode assembly of FIG. 2; while

FIG. 2B comprises side and front views of one of the electrodes of theelectrode assembly of FIG. 2A.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1 of the drawings, there is shown a cover unit forhermetically sealing a container for a semiconductor device whichcomprises a rectangular cover element 10, preferably of acobalt-nickel-iron alloy commercially available as KOVAR, and aplurality of contiguous turns of wire 11 of a heat-fusible material,preferably a gold-tin eutectic alloy comprising 80% gold and 20% tin.The turns of wire 11 are formed and disposed on the cover element 10with their outermost turn substantially in registry with the peripherythereof and the several turns are fused to the cover element 10 at aplurality of spaced points, preferably substantially at the corners asindicated by the dash lines 12. The dimensions of the cover element 10and the turns of wire 11 are greatly exaggerated for clarity ofillustration. While only four turns of wire are shown, in practice ithas been found preferable to provide approximately 20 turns of wire 11of about 0.00225 inch diameter while the cover element 10 is ordinarilyof a thickness of about 0.010 inch.

The apparatus for fabricating the sealing cover unit of FIG. 1 is shownin FIG. 2 and comprises a rotatable nonconductive supporting member 20mounted on a shaft 20a having a shallow cavity for receiving and closelyfitting the cover element 10. The apparatus also includes a mandrel 21rotatable with and engaging the cover element 10 in the recess of thesupporting member 20 and having an annular recess 21a proportioned toreceive a single-layer multiturn heat-fusible wire winding. The mandrel21 has an extending supporting shaft 22. The supporting member 20 andthe mandrel shaft 22 are rotated in unison by a motor 23 energized fromsupply terminals 24 through a switch 25. The motor 23 drives a shaft 26which, in turn, drives the rotatable supporting member 20 throughgearing 27 and the mandrel shaft 22 through gearing 28.

The apparatus further includes a nonrotatable electrode assembly 29shown in detailed perspective in FIG. 2A. This assembly includes acentral hole 29a through which the shaft 22 projects and a handle 29bfor raising or lowering the assembly. At the corners of the assembly 29are secured electrodes 30, 31, 32, and 33, the electrodes 30 and 31being connected in series with a direct-current source 34 and theelectrodes 32 and 33 being connected in series with a direct-currentsource 35. Each of the electrodes 30-33 is chisel-shaped as shown in theside and front views of FIG. 2B. The electrode assembly 29 normallydisengages the rotating elements 20 and 21 but the mandrel 21 has, ateach corner, a hole 21b for receiving one of the electrodes 30-33 of theelectrode assembly after the completion of the winding of the multiturnwinding and after the support 20 and mandrel 21 have come to rest.

Briefly, in the operation of the apparatus of FIG. 2, wire from a spool36 is attached to a point of the mandrel 21 and the switch 25 is closedto rotate the support 20 and the mandrel 21, forming a single layer ofwire winding in the recess 21a of mandrel 21. The wire is continuouslywound on the mandrel 21 until the periphery of the winding issubstantially in registry with the periphery of the cover, as shown inFIG. 1. When the winding is completed and the wire from the spool 36severed, the mandrel 21 and supporting member 20 are adjusted until theelectrodes 30-33 of the electrode assembly 29 register with the holes21b of the mandrel and the electrode assembly 29 is lowered so that thechisel-like welding electrodes 30-33 engage the winding thus formed andspot-weld each of the turns thereof to the cover element 10substantially at the corners of the cover element 10, as shown by thedash lines 12 of FIG. 1.

As stated above, the preferred material for the wire turns 11 is agold-tin eutectic alloy comprising 80% gold and 20% tin by weight. Thisalloy is very brittle. In order to make practical its fabrication, thewire must be formed and wound approximately 15° C. to 40° C. below themelting point of the gold-tin eutectic alloy which is about 280° C.Other solder alloys which are not brittle do not have the requirement ofbeing fabricated at an elevated temperature.

Coming back to the dimensions of the cover element 10 and the wire turns11, the cover element 10 preferably is of a thickness of about 0.010inch while the thickness T of the solder ring formed by the fusing ofthe wire turns 11 in the final hermetic sealing operation is desirablyabout 0.002 inch. In order to achieve this desired final thickness T,the diameter of the round wire of turns 11 must be 2T/√π or 1.128T; thatis, about 0.00225 inch. Obviously, if a wire of square cross-section isutilized, its thickness will be the same as the desired utlimatethickness of the solder ring.

While there has been described what is, at present, considered to be thepreferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein, without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

What is claimed is:
 1. A cover unit for hermetically sealing a containerfor a semiconductor device comprising:a metallic cover element; and aplurality of turns of wire of a heat-fusible material formed anddisposed on said cover element with the outermost turn substantially inregistry with the periphery thereof and said turns being fused theretoat a plurality of spaced points.
 2. An hermetic sealing cover inaccordance with claim 1 in which said cover element is acobalt-nickel-iron alloy and said wire is a gold-tin eutectic alloy. 3.An hermetic sealing cover in accordance with claim 1 in which saidplurality of turns of wire are contiguous.
 4. An hermetic sealing coverin accordance with claim 1 in which said plurality of turns of wire arespot-welded to said cover element at a plurality of spaced points.
 5. Anhermetic sealing cover in accordance with claim 1 in which said coverelement is rectangular and said plurality of turns of wire arespot-welded thereto substantially at the corners thereof.